1. Field of the Invention
The present invention relates to an image-taking lens apparatus and to a zoom lens system. More particularly, the present invention relates to an image-taking lens apparatus that optically takes in an image of a subject through a zoom lens system and then outputs it in the form of an electrical signal by means of an image sensor, and among others to a compact zoom lens system and to an image-taking lens apparatus provided with such a zoom lens system.
2. Description of the Prior Art
Recent years have seen increasing popularity of compact and portable data equipment terminals, such as cellular phones and personal digital assistants, that are furnished with a compact image-taking lens apparatus so as to be capable of inputting images. On the other hand, in devices (such as Web cameras) for inputting digital images, compact image-taking lens apparatuses are sought after. The image-taking lens apparatuses incorporated in such devices typically have, for reasons of data transfer speed, smaller numbers of pixels as compared with those designed for use in digital still cameras or the like that handle images with 4-million-pixels or higher resolution, which are quite common nowadays. As a result, when the magnification of an image is varied for trimming or the like, i.e., when an image is subjected to so-called electronic zooming, the image comes to have considerably poor quality. Thus, such image-taking lens apparatuses will benefit greatly from being provided with an optical zooming function, which does not degrade the quality of an image even when its magnification is varied. Inconveniently, however, incorporating an image-taking lens apparatus with an optical zooming capability in a compact device requires the image-taking lens apparatus to be miniaturized.
A compact image-taking lens apparatus is proposed, for example, in U.S. Publication No. 2003/0160902 A1. This publication discloses a zoom lens system that is so constructed as to include at least a first lens unit having a negative optical power and a second lens unit having a positive optical power (hereinafter a zoom lens system of this type will be referred to as a “negative-led zoom lens system”).
More specifically, the zoom lens system disclosed in the publication mentioned above includes a reflective optical element for bending the optical path. In this zoom lens system, bending the optical path helps make a camera slim, but additional inclusion of a thick optical low-pass filter and other extra components results in a comparatively long back focal length, hampering miniaturization of the entire system in terms of its length. Disposing an optical low-pass filter in the back focus region in this way makes it difficult to drastically shorten the back focal length. Moreover, shortening the back focal length causes the exit pupil to be located close to the image plane. As a result, off-axial rays exiting from the zoom lens system are obliquely incident on the image plane. This makes it impossible to fully exploit the light-condensing ability of the microlenses provided in front of a solid-state image sensor, resulting in extremely different brightness between in a central and a peripheral part of the image. This problem can be overcome by locating the exit pupil of the image-taking lens system away from the image plane, but doing so inevitably makes the zoom lens system as a whole unduly large.